This invention relates to an apparatus and method for controlling the amount of heat treatment given to a ceramic material which is being matured by heating. Specifically, it relates to the determination of when the ceramic material has received the proper amount of heat treatment, enabling a person or the apparatus itself to stop the heating process.
During the manufacture of ceramic articles, a firing or heating operation is necessary in order to cause the otherwise plastic ceramic material to maintain its shape. Heating is carried out (usually in a kiln) for lengths of time and at temperatures which will cause the ceramic object to reach a certain state of maturation which may be defined by various criteria such a porosity, density, or shrinkage. Regardless of which criterion is used, the ceramicist selects a condition of the material as "matured" and must then undertake to heat the material to, but not beyond, that point. While this invention is not concerned with defining the state of maturation of a ceramic material, it is concerned with regulating the heat treatment which the material receives so that the chosen state of maturation is attained efficiently. Too little heat treatment may result in a finished product which has insufficient strength or other undesirable qualities, while too much heat treatment wastes energy in heating the kiln and slows the rate at which the finished articles can be produced.
There are many apparatus for measuring the ambient temperature within a periodic kiln or for controlling the maximum temperature to which the kiln is heated or the rate at which the interior temperature rises. A typical apparatus of this type is described in Harkins, U.S. Pat. No. 3,901,437. The furnace operator selects both a maximum temperature and a rate of temperature increase (a "soak" period at maximum may also be provided), and the system activates the furnace heating components to cause the temperature to increase according to the selected values. Feedback is provided through a thermocouple which supplies a signal representative of kiln temperature.
It is apparent that this and similar apparatus suffer from the drawback that nowhere is an indication given of the state of maturation of the ceramic ware. The ceramicist using only these devices must therefore know beforehand, or through trial and error, what maximum temperature, rate of temperature increase, and soak time will mature a ceramic article to the desired degree.
It is well known in the ceramic arts that numbered pyrometric cones may be used during a firing operation to give an indication of the condition of the ceramic material. Cones neither measure nor control kiln temperature but react to the combination of temperatures and times to which they are exposed to provide an indication of the amount of heat treatment they have received. The indication provided by cones is their deformation, since they are composed of a fusible material. Consequently, a given cone may be used only once. This is the chief drawback of using pyrometric cones to indicate heat treatment.
Different values of heat treatment are indicated by deformation of different members of a particular set of cones (for example, the Orton Standard Pyrometric Cones manufactured by The Edward Orton, Jr. Ceramic Foundation of Westerville, Ohio). Also, for a given rate of temperature increase in the kiln, one may speak of a maturation temperature (for the piece being fired) or a deformation temperature (for the cone), rather than a value of heat treatment. Maturation or deformation temperature, however, is only meaningful in connection with a given rate of temperature increase. The deformation temperatures of some cones have been measured for various values of rate of temperature rise, and a few piecewise linear approximations of these relationships are shown in FIG. 1. Consequently, if a controlled rate of temperature increase can be maintained in a kiln (using, for example, the device of the Harkins patent), and if the desired degree of maturation of ceramic ware in the kiln is known to correspond to deformation of one of these cones, the operator may simply stop the firing operation at the proper deformation temperature and be assured of a matured ceramic article. Since an experienced kiln operator should easily be able to determine the proper cone number for a particular firing operation, the chief disadvantage of this method of determining when to end the firing is the need for an apparatus which can increase kiln temperature at a constant rate.
It is also possible to determine the end point of a ceramic maturing operation which is conducted at various heating rates, using only time and temperature information. This method makes use of the Arrhenius equation, a well-known relationship between time, temperature, and the extent of reaction. By integration, the duration of heating at various temperatures is reduced to an equivalent time at a single temperature; and if the maturation time at that temperature is known, heating can be stopped when the equivalent time equals the known maturation time. This system is described in Cooper, C. F., "Heat Work," 80 Trans. J. British Ceramic Society 60 (1981), and is also used for the curing of rubber, as explained, for example, in Smith, U.S. Pat. No. 3,819,915. Integration and calculation of an equivalent maturation time eliminate the need to control the rate of temperature increase; therefore, devices employing the Arrhenius equation may be used in spite of variations in the rate at which kiln temperature increases. Indeed, the rate need not even be measured or calculated. This advantage is achieved, however, only at significant cost. In the first place, one of the variables of the Arrhenius equation is activation energy. Any system based on the Arrhenius equation, therefore, requires the furnace operator to know, or discover through experimentation, the activation energy for the article being fired. Second, the nature of the calculation requires complex circuitry which not only carries out the mathematics of the equation and integrates equivalent times but also keeps track of kiln temperature at every instant during the firing.